1. Field of the Invention
This invention relates to quadrature couplers in general and more particularly to a quadrature coupler having a small package size.
2. Description of the Prior Art
A quadrature coupler is a four port device. A coupler separates signals based on the direction of signal propagation. These devices are used to unequally split the signal flowing in the mainline and to fully pass the signal flowing in the opposite direction. Referring to FIG. 1, a schematic diagram of a coupler is shown. Coupler 20 has an input port 22 and a through port 25 connected to coupled circuit line 30. An isolated port 24 and a coupled port 23 are connected to coupled circuit line 32. The lines 30 and 32 are broadside coupled lines that are coupled by an electromagnetic field. In an ideal situation some portion of the signal flowing into the input port 22 will appear at coupled port 23. Likewise any signal flowing into coupled port 23 will be coupled fully to input port 22. Isolated port 24 and input port 22 are isolated in that any signal flowing into port 22 will not appear at port 24 but will feed through to port 25.
For a 3 dB quadrature coupler, the amplitudes at coupled port 23 and through port 25 are 3 dB lower than at the input port 22. The phase difference between the input port 22 and ports 23 and 25 is ninety degrees. A differential (Zoo) and common (Zoe) mode impedance is associated with the quadrature coupler.
Couplers are typically mounted to a printed circuit board along with other electronic components. Prior art couplers have suffered from taking up excessive printed circuit board space. The space problem is worse when several couplers are required such as for a vector modulator or a balance amplifier. A quadrature coupler of the prior art has typical dimensions of 0.35 by 0.56 by 0.08 inches. These couplers are manufactured using a Teflon dielectric. Unfortunately, Teflon expands and contracts excessively due to temperature changes. Attempts have been made to make smaller couplers with Teflon. However, they have suffered from very poor electrical performance below 2 GHz.
Current couplers also do not allow the coupling to be adjusted after the coupler has been built.
Other types of prior art couplers have also been made with ferrite transformers along with appropriate resistors and capacitors arranged around the ferrite transformer. When these components are packaged together, they can take up a large amount of space. Placing resistors, capacitors and transformers complicates assembly using automated surface mount assembly equipment.
While couplers have been used, they have suffered from poor performance, a large size and in being difficult to assemble. A current unmet need exists for a coupler that is smaller, has high performance and is easily assembled.